1. Field of the Invention
This invention relates to curable epoxy resins having increased thermal shock resistance; and, more particularly, to certain anhydride cured epoxy resins containing a polyoxyalkylenepolyamine-maleic anhydride condensation product.
2. Prior Art
Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain desirable properties. One such class of curing agents are generally the anhydrides. The most commonly used anhydride curing agents are difunctional materials such as maleic anhydride, phthalic anhydride and the like, as well as tetrafunctional materials such as pyromellitic dianhydride.
Also known to be effective as epoxy curing agents or co-curing agents are various ureas and substituted ureas, such as those disclosed in U.S. Pat. No. 3,284,749, 2,713,569, 3,386,956, 3,386,955, 2,855,372 and 3,639,338. The ureas disclosed in the above references are useful as either a sole curing agent or as curing accelerators.
Aliphatic or aromatic compounds having a single terminal ureido group are well known. It has been disclosed in U.S. Pat. No. 2,145,242 to Arnold that diureido terminated aliphatic compounds can be produced by reacting an aliphatic diamine wherein each terminal amine has at least one labile hydrogen with urea. Other substituted ureas are disclosed in U.S. Pat. No. 3,965,072.
Epoxy resins for casting, embedding or encapsulating etc. must withstand repeated cycles of high and low temperatures without cracking. However, lowering the temperature increases stress due to shrinkage and reduces the ability of the resin to flow, thus relieving the stress.
Anhydride cured resins are useful in applications where high heat deflection is required. However, such materials exhibit brittleness and thus a low resistance to thermal shock. Diluents and modifiers do improve thermal shock resistance properties but, unfortunately, adversely influence the heat deflection properties as shown in May and Tanaka, Epoxy Resins, New York, 1973, p. 299. Likewise, plasticizers have not found wide acceptance in epoxy technology primarily because most of them are incompatible with the cured resins.
It has now been unexpectedly found that polyoxyalkylenepolyamine-maleic anhydride condensation product having thioureide, ureide or thioformamide or formamide end groups, when employed as an epoxy additive, provides cured epoxy resin compositions exhibiting outstanding thermal shock resistance. Specifically, epoxy resins incorporating these additives, upon curing with a specific bicyclic anhydride curing agent, provide a material with high heat deflection and superior resistance to thermal shock.